Ningxia Dadi PVA 1788

Ningxia Dadi PVA 1788 is a partially hydrolysed polyvinyl alcohol resin supplied as a white to off-white granular powder. The grade designation encodes the polymer architecture: the first two digits, 17, indicate an average degree of polymerisation close to 1700, while the final two digits, 88, indicate a target alcoholysis degree of 88 mol%. Residual acetate groups remaining after alcoholysis of polyvinyl acetate interrupt interchain hydrogen bonding and crystallite packing. The material is produced by continuous alcoholysis of polyvinyl acetate in a methanol solution using sodium hydroxide as the saponification catalyst. The chemical abstract registry number is 25213-24-5. Because the grade is partially hydrolysed rather than fully hydrolysed, the polymer chain retains a narrow distribution of vinyl acetate units that modify cold-water hydration, interfacial activity and film flexibility. The product is used in emulsion polymerisation, paper surface sizing, textile warp sizing, remoistenable adhesives and water-soluble packaging films. Industrial users should verify finished-article regulatory compliance under EU 10/2011, FDA 21 CFR 175.300 or REACH Article 33 as applicable; the resin itself is not an electrical and electronic equipment material and therefore falls outside the scope of RoHS Directive 2011/65/EU.

Typical release ranges and test designations for the grade are given in Table 1.

Packaging is normally 25 kg multi-wall paper bags with an inner polyethylene liner. Storage below 30 °C and relative humidity below 70 % is required. If moisture content exceeds 5.0 %, pre-drying at 80–90 °C for 2–4 h in a forced-air tray dryer restores handling performance. Avoid storage adjacent to strong oxidizers and concentrated acids.

Processing behaviour under high-shear aqueous dissolution is governed by degree of hydrolysis and residual acetate content

Dissolution of PVA 1788 is not a simple hydration step. Because the resin is partially hydrolysed, cold water at 20–30 °C initially disperses the granules and begins swelling, but full disentanglement requires thermal energy. A common stock concentration is 4–10 wt%. The powder is added slowly to the vortex of a stainless-steel jacketed vessel fitted with an axial turbine or anchor agitator at a tip speed of 2.0–4.0 m/s. The slurry is heated to 85–95 °C and held for 30–60 min. After reaching 85 °C, the mixture develops a clear or slightly hazy solution. If the powder is charged directly into hot water above 80 °C, the outer particle surface hydrates so rapidly that a gel skin forms, trapping undissolved granules. Excessively long hold times at 95 °C increase foam stabilisation because the residual acetate groups act as interfacial tension modifiers; defoamer addition of 0.05–0.2 % by weight is often necessary in high-shear mixing.

For high-shear rotor-stator dispersion, published data for this specific grade in that configuration is limited; plant experience indicates that rotor-stator devices above 3000 rpm generate local temperature excursions that reduce final solution clarity unless the vessel is jacket-cooled to maintain bulk temperature below 95 °C. At 4 % and 20 °C, the resulting solution viscosity is 20.0–26.0 mPa·s. At 10 %, viscosity is not linearly scaled; pseudoplastic behaviour is observed under Brookfield rotational measurement. pH drift during dissolution should be checked: if pH falls below 4.5, residual sodium acetate contamination or carbon dioxide absorption is indicated and the batch may require trace alkali correction. A 4 % solution stored at 20–25 °C in sealed tanks is normally usable for 48–72 h before viscosity drift exceeds 10 %; biocide is required beyond 48 h.

In vinyl acetate and vinyl acetate-ethylene emulsion polymerisation, PVA 1788 is employed as a primary protective colloid between 2.0 wt% and 4.0 wt% based on total monomer. It is dissolved in the aqueous phase before monomer addition. In a standard baffled reactor with a 0.35–0.50 W/kg agitator power input and dual pitched-blade turbine, the resulting vinyl acetate homopolymer emulsions typically develop a particle size of 0.3–1.0 µm and Brookfield RVT viscosity of 1000–4000 mPa·s at 50 % solids; measurement of particle size follows ISO 13320:2020. The partially hydrolysed structure provides better interfacial packing than fully hydrolysed grades, but less aqueous shear stability than high-molecular-weight polyvinyl alcohols with added nonionic surfactants. The residual acetate groups promote grafting at 60–80 °C, which increases latex viscosity during hold periods. Overdosing above 4.0 wt% may raise coarse grit through bridging flocculation; the limiting factor is reactor fouling rather than polymer solubility. Film formation after drying occurs without added plasticizer; elongation at break measured on a cast film conditioned at 23 °C and 50 % RH according to ASTM D882-18 is generally 150–250 % for 88 mol% hydrolysed PVA grades. Published data for Ningxia Dadi PVA 1788 in this precise test configuration is limited.

Where does this grade diverge from PVA 1799 and PVA 2488 in adhesive and emulsion systems?

The comparative distinction is driven by two variables: degree of hydrolysis and degree of polymerisation. PVA 1788 and PVA 1799 share a similar nominal degree of polymerisation of 1700, but differ in residual acetate content. PVA 1799 is fully hydrolysed to 98.0–99.0 mol%; its interchain hydrogen bonding is denser, producing stronger films and lower cold-water solubility, but it requires dissolution temperatures above 90 °C and forms higher-viscosity solutions at equivalent concentration. PVA 2488 carries the same 86.0–89.0 mol% hydrolysis range as PVA 1788 but has a higher average degree of polymerisation of 2400; its 4 % solution viscosity is typically 44.0–52.0 mPa·s, roughly double that of PVA 1788.

In adhesive systems, PVA 1788 provides moderate tack and open time. For water-resistant joints, a crosslinker such as glyoxal at 2.0–5.0 wt% on dry PVA reduces rewetting. PVA 1799 is selected where slow water dissolution and high static load resistance are valued; PVA 2488 is selected where higher solution viscosity and film toughness are required but with higher mixing torque. The lower viscosity of PVA 1788 permits a 2–4 % higher solids level in the same mixer torque envelope. In borated dextrin adhesives, free borate above 0.1 wt% causes viscosity build through polyol complexation; maintain pH above 8.0 and add PVA as a pre-dissolved 10 % solution to avoid local gelation.

Film formation, cold-water resistance, and viscosity build in warp sizing

Textile warp sizing uses PVA 1788 as a film-forming binder blended with starch or acrylic size. In a typical size-mix vessel, PVA 1788 is cooked at 85–95 °C with starch for 30–60 min. The size box viscosity is controlled between 6.0 s and 12.0 s using a Zahn cup No. 2 at 85 °C. Dry pickup on cotton/polyester yarn is usually 8.0–12.0 % by weight. The size film after conditioning at 23 °C and 50 % RH exhibits tensile strength in the 25–35 MPa range and elongation at break above 200 % according to ASTM D882-18; published data for the specific Ningxia Dadi product in a converted size film without starch is limited. The partial hydrolysis level gives enough water solubility for desizing in hot water at 70–85 °C without requiring enzymatic desizing, unlike fully hydrolysed PVA 1799 which may leave residues if desizing temperature falls below 90 °C.

For paper surface sizing, PVA 1788 is applied at 3.0–6.0 wt% solids through a size press or film press at 50–60 °C. The lower aqueous viscosity at machine temperature improves transfer uniformity and reduces filming defects compared with PVA 2488, while the residual acetate groups maintain film flexibility. Dry pickup is typically 0.4–1.2 g/m²; Cobb 60 water absorption measured according to ISO 535:2023 and IGT surface strength measured according to ISO 3783:2006 are used to confirm performance. When water resistance must exceed that of PVA 1788 alone, a blend of 70–80 wt% PVA 1788 and 20–30 wt% PVA 1799 is used; this blend raises the dissolution temperature but reduces sizing dusting. In remoistenable adhesive coating, the low ash and controlled pH minimise interference with borated dextrin; viscosity drift in the coating pan is typically below 0.3 mPa·s over 4 h at 40 °C in closed containers.

When the grade is substituted into a fully hydrolysed PVA line, pre-drying and storage-time controls become the critical variables

Substitution of PVA 1788 into a process designed for PVA 1799 is not directly neutral. Because PVA 1788 hydrates at lower temperatures, dry blending lines that discharge into hot water at 80 °C may form agglomerates before the batch reaches full cook temperature. The preferred procedure is to add PVA 1788 to the cold-water phase first, then raise the temperature under agitation. In twin-screw extrusion of water-soluble film, moisture control is more stringent: pelletised PVA 1788 with moisture above 2.5 % leads to melt-phase bubbles and dimensional variability; a vented twin-screw extruder with a 25:1 L/D ratio and vacuum venting at −0.06 to −0.08 MPa gauge is typically used. Published data for this specific Ningxia Dadi grade in water-soluble blown film extrusion is limited; pilot trials are required to set the temperature profile between 160 °C and 200 °C because residual acetate groups lower the onset of thermal decomposition relative to fully hydrolysed grades.

In aqueous adhesive compounding, the lower solution viscosity of PVA 1788 means that direct substitution of PVA 1799 at equal weight will reduce final viscosity; the reduction is governed by the 4 % solution viscosity difference of 20.0–26.0 mPa·s for PVA 1788 versus 25.0–31.0 mPa·s for PVA 1799. Compensation with PVA 2488 or higher solids may be required. The hydrolysis difference also reduces wet blocking resistance; published data for blocking loads of Ningxia Dadi PVA 1788 under this specific conditioning is limited.